Current role: Senior software engineer with a service based company
Experience: ~9 YOE

Recruiter contacted about a role in productivity engineering department of LinkedIn and I was interested to proceed with it.

Online Assessment:
I generally do leetcode to understand the differenct approaches of problem solving. Never wanted to memorize the solution for the sake of interviews.

I was asked 2 problems.

1. Popular substring problem (Medium)
   Applied sliding window approach. Explained the approach and wrote the code for the approach in 15 minutes- but had used built in methods like substring in that code. Then the interviewers say they wouldnt want to see built in functions used. Refactored it to use hashmap and this took another 15 minutes for explanation and writing code. We then ran thro the code for the different inputs - took another 10 minutes.
2. Non leetcode easy problem
   Solved it with a sub-optimal approach in 10 minutes and optimized it in 5-7 minutes.

I was confident that I would be moved to the next round as I had given optimal solutions to both the problems but a day later the recruiter emails that I have been rejected. I was disappointed and now questioning my own approach to study for the other interviews that I have got lined up.
Is the expectation to memorize and give the optimal solution in the first try? Why is iterative optimization no longer accepted?